The present invention relates to a closed-loop electronic control system for controlling combustion in a diesel engine operating with highly premixed combustion.
As is known, in the execution of the thermal cycle of a diesel engine reactions of oxidation of the fuel take place, in which the conditions of ignitibility of the fuel mainly depend upon the temperature, the pressure and the concentration of oxygen in the combustion chamber. Combustion is regulated by acting on the law of injection (amount of fuel injected per unit time), regulating the ignition delay and the amount of fuel that is accumulated in the chamber prior to the start of combustion.
In particular, in the process of combustion it is possible to highlight two distinct steps, namely:                a first step referred to as premixing step, in which the fuel accumulated prior to ignition is burnt; and        a second step referred to as diffusion step, in which the fuel injected subsequent to the start of combustion is burnt.        
In the first, premixing, step the fuel is self-ignited after having had time for mixing with the gas present in the combustion chamber.
The quality of said mixing depends upon the ignition delay, upon the form of motion of the gas in the chamber (turbulence, swirling motion, etc.) and upon the characteristics of the spray of fuel.
The mixing times are in any case much shorter than the ignition delay in the cases of common application, and therefore substantially all the fuel injected prior to the start of combustion is premixed.
The high degree of premixing enables the fuel to use the oxygen present in the chamber as well as possible, guaranteeing complete combustion without any emission of smoke. The step of premixed combustion is, however, responsible for combustion noise.
The diffusion step is instead characterized in that the fuel is injected into the chamber after combustion has started and hence in the presence of very high temperatures and pressures. In said conditions, the ignition delays are minimal, and the fuel is able to burn albeit in the presence of very low amounts of oxygen. It occurs, however, that part of the fuel, on account of the low degree of mixing, will not succeed in initiating or completing the reactions of combustion owing to the lack of oxygen with which to react, but at the same time will be heated considerably by the heat produced by the combustion in the chamber, leading to the formation of solid particles in the form of soot. The low degree of mixing is at the same time responsible for a greater production of nitrogen oxides (NOx) in so far as it leads to the formation of areas in the chamber more involved in the combustion, which remain at high temperatures for a long time (the so-called hot points), in which the NOx are more easily formed. The diffusion step is then responsible for the formation of the majority of the emissions of smoke and nitrogen oxides. At the same time, it is, however, characterized by a progressive release of energy, regulated by the law of injection of fuel, which generates low pressure gradients in the chamber and hence low combustion noise.
It is moreover known practice, for the purpose of regulating the noise and the emissions, to carry out a first injection, referred to as pilot injection, and at least one subsequent injection, referred to as main injection.
In particular, the pilot injection concurs in dividing combustion between a premixing step and a diffusion step. In fact, as the amount of fuel injected via the pilot injection increases, the ignition delay is reduced, and the amount of accumulated fuel to be burnt in the premixing step is reduced at the same time, thus leading to a lower combustion noise but an increase in pollutant emissions.
Conventional combustion is characterized by a compromise between emissions and combustion noise, which, once it has been defined in the design stage, must be as repeatable and reliable as possible, cylinder by cylinder, throughout the life of the engine.
For example, the patent applications EP-921.296 and DE-102.21.001 propose solutions in which the amount of fuel supplied using the pilot injection is adjusted in order to control the combustion noise and the instant of ignition. The patent EP-921.296 proposes the use of a noise sensor; and the patent DE-102.21.001 proposes the use of an ionization-current sensor, which is able to detect the start of combustion.
Furthermore, in order to reduce the emissions of NOx use is commonly made of the recirculation of burnt gases in the combustion chamber. This is known as exhaust-gas recirculation (EGR). According to this practice, there is introduced into the chamber a gas, the carbon dioxide present in the products of combustion, which is characterized in that it has a greater thermal capacity and hence is able to reduce the temperatures in the combustion chamber given the same heat developed by combustion, and is able to reduce the heat exchange between the hot areas of the chamber, with the overall effect of reducing the nitrogen oxides as a whole produced by combustion. Said amount of recirculated burnt gases reduces, however, the amount of oxygen in the chamber and hence, given the same degree of mixing of the gases, causes an increase in the emissions of smoke. The total amount of recirculated exhaust gases is usually controlled by means of a closed-loop control using an air-flow sensor.
As an alternative to the conventional diesel combustion outlined above, of considerable interest in the field of diesel engines is premixed-charge compression ignition (PCCI), which derives from the idea of premixing, either completely or to a very high extent, fuel and gas prior to the start of combustion, this tending towards the combustion of a homogeneous mixture and eliminating the disadvantages of diffusive combustion.
PCCI combustion is however possible only if the ignition delays are greater than the duration of the injection event and can be used only if it is possible to control the energy release of the premixed mass of fuel at the moment of ignition, in order to prevent damage to the engine and to control combustion noise.
In the case where it is possible to obtain combustion of a totally premixed mixture low emissions of smoke and NOx are obtained.
A series of studies has demonstrated the great difficulty involved in using PCCI combustion in all the ranges of use of the engine, when this is oriented towards an automotive use. This behaviour is highlighted in FIG. 1, which illustrates the operating map of a conventional diesel engine (solid line) and the operating map of a diesel engine running with PCCI combustion (dashed line) in the engine diagram defined by the engine r.p.m. (RPM) and the brake mean effective pressure (BMEP).
The behaviour highlighted above, i.e., the impossibility of using the plane of operation of the engine completely, has led to the development of strategies (for example the ones described in WO 98/07973, U.S. Pat. No. 5,875,743, and EP-0997.624), in which the use of PCCI combustion is confined to certain given regimes and operating loads of the engine.
PCCI combustion is moreover characterized by a degree of instability due to the fact that, since long ignition delays are generated in order to obtain a high degree of premixing, start of combustion is uncoupled from phasing of injection, thus rendering problematical direct control of the start of combustion and of the engine angular position.
At the same time, in order to reduce energy release at the moment of ignition, for the purpose of preventing damage to the engine and excessive combustion noise, in the majority of applications recourse is had to a large amount of recirculated burnt gases, which may even reach 70% of the total mass of gas in the combustion chamber. In such conditions combustion risks degenerating into a misfire, at a minimal variation of the control parameters, the most important being the mass of recirculated exhaust gases, thus contributing to an increase in the potential instability in combustion.
Direct control of combustion applied to PCCI combustion hence becomes an indispensable tool for the very operation of the engine. Examples of documents that propose a direct control of combustion are the U.S. patent applications US20020007816, U.S. Pat. Nos. 6,637,404, 6,142,119, US20030230276, and US20020011240.
The purpose of the present invention is to provide an efficient and simple control strategy for PCCI combustion.
The above purpose is achieved by the present invention in so far as it relates to a closed-loop electronic control system for controlling combustion in a diesel engine operating with highly premixed combustion, in which an injection system is designed to obtain at least two fuel injections in one cylinder per engine cycle, said injections comprising at least one pilot injection and a subsequent main injection, said system being characterized in that it comprises:                sensor means designed for measuring or calculating quantities characteristic of the process of fuel combustion; and        closed-loop control means, which carry out a control of the modality of fuel injection based upon the measured or calculated value of said quantities and upon reference values by regulating at least one of the following parameters:        
a) the distance in time between the pilot injection and the main injection; and
b) the amount of fuel injected by means of the pilot injection.